Adding the numbers that are stored as strings

public static string CombineNumbers(string number1, string number2)
    {
        int length = number1.Length > number2.Length ? number1.Length : number2.Length;
        string returnValue = string.Empty;

        for (int i = 0; i < length; i++)
        {
            int n1 = i >= number1.Length ? 0 : int.Parse(number1.Substring(i,1));
            int n2 = i >= number2.Length ? 0 : int.Parse(number2.Substring(i,1));
            int sum = n1 + n2;

            returnValue += sum < 10 ? sum : sum - 10;
        }
        return returnValue;
    }